1. Field of the Invention
The present invention relates generally to measurement and instrumentation systems and, more specifically, to a scanning front end using single-pole, double-throw (SPDT) switches to multiplex input signals.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a board or module plugged into one of the I/O slots of the computer system, or a board or module plugged into a chassis, or an external device. Generally, the computer system has an I/O bus and connectors or slots for receiving I/O boards. Various computer systems and I/O buses may be used to implement a processing system for receiving the measurements. The computer may receive measurements from multiple sources, where, for example, different sources may interface with the computer through respective input and output measurement modules.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a UUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in an analog input module of a DAQ system. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an analog-to-digital converter (ADC), and transmitted to a computer system for storage and/or analysis. In addition, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in an analog output module of the DAQ system. The DACs convert the digital signal to an analog output signal that is used, e.g., to stimulate a UUT.
Most scanning front ends on analog input modules use some type of “N to 1” multiplexer followed by a signal conditioning stage (usually an operational amplifier). There are at least two drawbacks to this topology. First, multiplexers have some parasitic capacitance associated with their output node. When switching from one channel to the next, this capacitance transfers charge between channels. This charge transfer may cause the equivalent input impedance of the multiplexer to decrease as the sampling frequency increases. Second, since the voltage at the output of the multiplexer may be changing as different channels are selected, the common mode voltage at the signal conditioning stage may follow these input voltages. The common mode voltage following the input voltages may prevent the signal conditioning circuitry, e.g., the amplifier, from settling quickly when switching from one channel to another.